Electrolysis

6 marks · Normal

A student sets up an electrolysis experiment using molten potassium iodide (KI). An external circuit with a bulb lights up when the current is switched on.

(a)

State the ions present in molten potassium iodide. Identify which ions move towards the cathode and which move towards the anode during electrolysis.

[2 marks]

(b)

Write the ionic half-equation for the reaction occurring at the cathode.

[1 mark]

(c)

Write the ionic half-equation for the reaction occurring at the anode.

[1 mark]

(d)

For the electrode reaction at the cathode, state whether it is an oxidation or a reduction. Justify your answer using the concept of electron transfer.

[2 marks]

Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.

Show answer

(a) Ions: K^+ and I^-. K^+ ions move to the cathode, I^- ions move to the anode.

(b) K^+ + e^- → K

(d) Reduction occurs at the cathode because potassium ions gain electrons (electron transfer: gain of electrons is reduction).

Common mistakes: Halving or doubling the number of electrons incorrectly in half-equations; Wrongly identifying which electrode does oxidation or reduction

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · States both correct ions present (K^+ and I^-)

B1 (a) · 1 mark · Identifies which ion moves to each electrode (K^+ to cathode, I^- to anode)

B1 (b) · 1 mark · Correct ionic half-equation for cathode (K^+ + e^- → K)

B1 (c) · 1 mark · Correct ionic half-equation for anode (2I^- → I2 + 2e^-)

B1 (d) · 1 mark · Correctly identifies the cathode reaction as reduction

B1 (d) · 1 mark · Justification includes gain of electrons (reference to electron transfer)

Worked solution

a) The ions in molten KI are potassium ions (K^+) and iodide ions (I^-). During electrolysis, K^+ ions move to the cathode (negative electrode), and I^- ions move to the anode (positive electrode).

K^+ to cathode, I^- to anode.

b) At the cathode, the potassium ions gain electrons to form potassium metal:

c) At the anode, iodide ions lose electrons to form iodine molecules:

d) The cathode reaction is a reduction because potassium ions gain electrons (reduction is gain of electrons).